Put tea bags in a container that will hold the 3 cups of water. Boil the 3 cups of water, and when they come to a boil, pour the boiling water over the teabags. Let the bags steep for at least 3 minutes. Remove the tea bags and let the tea cool.
Once cooled, place it in the refrigerator. You can do this a day ahead or a few hours before you are going to make the cooler.
Blend the watermelon chunks in until it is like a smoothie. In a pitcher that holds at least 6 cups of water, add the tea, simple syrup, lemon juice, and melon smoothie. Stir it all together until nicely mixed. Add some crushed ice when ready to serve.
If it is too tangy, you can add more crushed ice or cold water.